Sound attenuating structure

ABSTRACT

A sound-attenuating structure includes a non-absorbing shell and a mat of sound-absorbing material held parallel to and spaced from the shell a distance equal to one-quarter the wave length of the highest frequency to be attenuated. The thickness of the mat of sound-absorbing material is defined by the expression:

United States Patent 11 1 Sorber 1 July 17, 1973 SOUND ATTENUATINGSTRUCTURE [75] Inventor: Siegfried Sorber,Wolfsburg,

Germany [73] Assignee: Volkswagenwerke Aktiengesellschaft, Wolfsburg,Germany 221 Filed: Dec. 15, 1971 21 Appl. No.: 208,227

[30] Foreign Application Priority Data 2,059,487 11/1936 Peik 181/422,271,892 2/1942 Bourne 181/48 2,311,676 2/1943 Maxim 181/48 2,326,6128/1943 Boume 181/33 D 3,353,626 l-1/1967 Cremer et a1 181/42 FOREIGNPATENTS OR APPLICATIONS 288,014 10/1966 Australia 181/55 1,226,4382/1960 France 181/59 733,329 7/1955 Great Britain 181/48 PrimaryExaminer-Richard B. Wilkinson Assistant Examiner-John I". GonzalesAttorney-(.iranville M. Brumbaugh. Ronald B. Hildreth et all.

[57] ABSTRACT A sound-attenuating structure includes a nonabsorbingshell and a mat of sound-absorbing material held parallel to and spacedfrom the shell a distance equal to one-quarter the wave length of thehighest frequency to be attenuated. The thickness of the mat ofsound-absorbing material is defined by the expression:

l U2 f|)l 'f1'f2 where c is the propagation speed of sound in theatmosphere, f is the highest, and f, the lowest acoustic frequency to beattenuated.

10 Claims, 5 Drawing Figures Patented July 17, 1973 FIG. 2

zorfumommd oZDOm uO mumowo in KH FREQUENCY FIG. 3 6. 5

SOUND ATTENUATING STRUCTURE BACKGROUND OF THE INVENTION This inventionrelates in general to sound-attenuating structures, and in particular,to an application in the automobile industry as a sound-attenuating firewall between the engine and passenger compartments, and also as a fluidconduit such as an exhaust pipe.

In known sound-attenuating arrangements, it is a common expedient toapply a sound-absorbing mate rial to a non-absorbing supporting shellsuch as a sheet metal partition. While this technique may indeed achievethe desired sound attenuation, provided the sound-absorbing layer isthick enough, it is a wasteful technique because a substantialproportion of the sound-absorbing material contributes nothing to thedesired sound-attenuating effect but merely adds size, weight and costto the final product. What the art lacks and has long sought is asound-attenuating structure which uses the sound-absorbent material atmaximum efficiency so that the size, weight and cost of the finalproduct is minimized.

SUMMARY OF THE INVENTION Accordingly, there is provided, according toone embodiment of this invention, a sound-absorbing wall having anon-absorbing shell and a sound-absorbing mat spaced from thenon-absorbing shell a distance equal to one-quarter of the wave lengthof the highest frequency to be attenuated.

Another embodiment of the invention is a fluid conduit in which thenon-absorbing shell forms the exterior casing of the conduit, and thespaced sound-absorbing mat is disposed interiorly of the shell andconcentrically therewith, leaving an-interspatial zone between the shelland the mat for flow of fluid.

To achieve maximum efficiency, the thickness d of the mat of absorbentmaterial may be calculated by the following relationship:

d I(fz fi)] 'f1'f2 where c equals the sound propagation velocity in theatmosphere and f, and f, equal the highest and lowest frequencies to beattenuated, respectively.

DESCRIPTION OF THE DRAWINGS For a better understanding of the invention,reference may be had to the following description of a preferredembodiment, read in conjunction with the figures of the accompanyingdrawings, in which:

FIG. 1 is a cross section of a portion of a curved sound-absorbing wallconstructed according to this invention;

FIG. 2 is a graph in the solid curve showing the percentage of soundabsorption at various frequencies for a given thickness ofsound-absorbing material applied directly to a non-absorbent shell, andshowing, in the broken curve, the corresponding percentage of soundabsorption with the same thickness of absorbent material spaced from thenon-absorbent wall according to the present invention;

FIG. 3 is a cross section of a conduit constructed according to thepresent invention;

FIG. 4 is a cross section of a second conduit constructed according tothe present invention; and

FIG. 5 is a cross section of a third conduit constructed according tothis invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawingswherein like characters designate identical or corresponding parts, andmore particularly to FIG. 1 thereof, a sound-absorbing wall constructedaccording to the invention is shown having a sheet metal shell 10 and asound-absorbing acoustic panel or mat l2 spaced therefrom a uniformdistance b by spacer webs 14. The sound-absorbing mat 12 may be porousor open pored material, for example, or it may alsobe fibrous material.It may be compact homogeneous material or it also may be compositematerial formed of individual strands of sound absorbing material boundor adhering together and providing interstices or dead air spacesbetween the strands to con tribute to the sound absorbing properties ofthe material. The shell 10 is typically sheet metal as in the usualconstruction of a fire wall between the passenger and enginecompartments of an automobile. Shell 10, however, may be of any othersuitable material such as plaster board or plywood as the requirementsof different applications dictate. The spacer webs 14 are preferably asfew as possible and just enough to hold the acoustic mat 12 in position.The spacer webs 14 may take any known form and are shown schematicallyfor purposes of illustration.

The interspatial dimension b between the inside face 16 of the acousticmat l2 and the outside face 18 of the shell 10 is selected to coincidewith one-quarter of the wave length of the highest frequency to beattenuated. For example, there is little utility in attenuating thefrequencies that lie above the normal hearing range of a human being orfrequencies which are within the hearing range but, because of theconditions in which the invention is used, are at low amplitudes or areotherwise not objectionable.

The theory which is believed to explain the results achieved by thisinvention is described below. This theory is offered to furtherillustrate the construction of the preferred embodiments and should notbe con strued as a limitation to the appended claims.

Adjacent the non-absorbing shell 10, the acoustic particle velocitynormal to the shell is zero so that sound absorbing material placedthere, as it is in the prior art, does not contribute to soundabsorption. This is because sound absorption is related to the acousticvelocity; the particle velocity at the particular location in the soundwave is attenuated by conversion into heat generated by viscous lossesof the moving particles of the sound carrying medium within thecapillary pores of the porous material, and by friction losses of thevibrating component parts of the material. Since the acoustic velocityadjacent the non-absorbing shell is zero, there is no contribution atthat location to the sound attenuation. Accordingly, acoustic materialplaced at that location in the sound wave does not contribute to soundattenuation and merely represent wasted weight and expense.

On the other hand, the acoustic velocity is maximum at a distance fromthe shell 10 equal to one-fourth the wave length of the highest acousticfrequency to be absorbed. At this point, the sound absorption materialhas the highest efficacy since that is the point of greatest amplitudeof the oscillations of the particles in the sound carrying medium.

The improved sound attenuation contributed by this invention isillustrated in FIG. 2 which is a graph of the relationship between thesound absorption value, measured in percent, and the acoustic frequency,measured in kilohertz for a given material. The. curves reproduced havebeen plotted for a given static air pressure, a given air temperatureand mean thickness of the sound-absorbing material.

Examining first the full-line curve which represents the case in whichthe sound-absorbing material is applied directly on the non-absorbingwall with no intermediate space provided therebetween, i.e., the stateof the art, we find a maximum sound absorption degree at a frequencyabove 4 kHz. If a separation in accordance with the invention is thenintroduced between the sound-absorbing material and the wall, measuring,for example, 1.2 cm, the full-line curve is shifted into the curveillustrated by the dotted line. It is obvious that now the maximum ofthe sound absorption curve is placed between two and 3 kHz.

As a role, however, it is not sufficient to attenuate a single frequencyor a very narrow frequency band but rather an acoustic frequency band ofa particular width must be absorbed in the material. Whereas theinterspatial dimension b between the non-absorbing partition and thesound-absorbing panel serves to determine the highest acoustic frequencyattenuated, the thickness of the sound-absorbing panel constitutes afurther parameter which may be utilized to determine the acousticfrequency range to be absorbed. In a given application, the acousticfrequency range will be f -f wherein f constitutes the highest and f thelowest acoustic frequency to be attenuated. For a given frequency range,the thickness d of the acoustic panel should be d i (f2 fl)]/ 'f1f2where c is the sound propagation velocity in the ambient medium. In mostcases, the medium present will be air; however, the invention may alsobe utilized in other fluid mediums having different values of c, inwhich case the thickness d will change proportionately.

The sound attenuating structure may also be constructed as a conduitwherein the shell forms the outside circumferential casing of theconduit and the mat is disposed coaxially within the shell and spacedfrom the interior surface thereof to leave an annular interspatial zoneof uniform cross sectional width. This annular interspatial zone may beused as the fluid flow zone to convey the fluid which flows in theconduit.

The acoustic mat disposed coaxially in the'conduit may take the form ofa core 19 of solid cross section, as shown in FIG. 3. In thisembodiment, the interspatial zone 24 between the interior surface 20 ofthe shell and the exterior surface 22 of the acoustic mat forms a freeflow zone 24 for conveyance of fluid in the conduit. Placement of theflow zone between the mat and the shell yields the advantage that thecross sectional area of the free flow zone 24 becomes larger than inknown attenuation devices wherein the same thickness of the mat isapplied directly to the shell. As a result, the velocity of flow and thesound level become lower. Moreover, due to the concentration of thesoundabsorbing material in the area of the conduit axis the effectivethickness of the material is increased since it serves to absorb soundfrom both diametrical sides of the conduit and the exterior dimensionsof the conduit remain the same while the quantity of material isreduced.

The sound absorbing material 19 may be subdivided axially bytransversely extending partitions 23 held to the wall of conduit 10 andby means of webs 14. The partitions 23 are for the purpose of preventingacoustic propagation within the absorbing material in the direction ofthe conduit axis and, in order not to adversely effect the free flowsection 24, the web 14' is made narrow in the direction of the conduitaxis.

As shown in FIG. 4, the acoustic mat may take the form of an annulus 25coaxially disposed with respect to the shell 10. In this embodiment, thecross sectional thickness d of the panel is defined by the sameexpression which defines the mat thickness of the sound attenuating wallshown in FIG. 1. In this embodiment the axial zone 26 is used forconveyance of fluids through the conduit while the interspatial zone 24may be used for fluid conveyance or may be divided into hollow chambersby partitions 23' which also divide the sound absorbing mat 25 intopieces to prevent sound propagation axially through the mat 25. If theinterspatial zone 24 is to be used for conveyance of fluid, then thedividers 23 will not extend out to the conduit wall 10 but will extendonly through the mat 25 and will be connected to the conduit wall 10 byconnecting webs similar to webs 14 in the embodiment of FIG. 3.

As shown in FIG. 5, the mat 19' may extend laterally to contact theconduit 10 on diametrically opposite sides thereof. In this embodimentthe cross sectional area available for fluid flow is less than theembodiment of FIG. 3, but the mechanical strength of the arrangement andthe simplicity of manufacturing techniques make this a desirableembodiment for some applications.

The embodiments of the invention disclosed herein provide soundattenuating structures which arrange the sound absorbing material tominimize weight, size and cost and make the formerly wasted spaceavailable for useful purposes. The quantity of sound absorbing materialused in these embodiments is less than that used in prior artstructures, yet provides sound absorbing characteristics that are animprovement over the prior art.

Obviously, numerous variations and modifications of the above-describedpreferred embodiments of the invention may be made in light of thisdisclosure. It is therefore to be expressly understood that theinvention may be practiced otherwise than as specifically describedwhile still remaining within the scope and spirit of the claims appendedhereto which define the invention.

I claim I. A sound attenuating structure, comprising;

a shell;

a mat of sound-absorbing material disposed adjacent and parallel to saidshell and spaced therefrom a distance substantially equal to one-fourthof the wave length of the highest frequency to be attenuated; and

means for holding said mat parallel to said shell.

2. A sound attenuating structure, comprising:

a shell;

a mat of sound absorbing material disposed adjacent to said shell andspaced therefrom a distance sub stantially equal to one-fourth of thewave length of the highest frequency to be attenuated, said mat having athickness d having a value approximated by the expression:

where f is the highest acoustic frequency to be attenuated, f is thelowest acoustic frequency to be attenuated, and c is the soundpropagation velocity in the ambient medium.

3. A sound attenuating structure, comprising:

a shell forming the exterior wall of a conduit;

a mat disposed interiorly of and adjacent to said shell and spaced fromthe interior surface thereof a distance substantially equal toone-fourth of the wave length of the highest frequency to be attenuated;and

means for holding said mat parallel to said shell.

4. A sound attenuating structure as defined in claim 3, wherein said matforms a solid core filling the axial portion of said conduit, the matand the interior surface of said shell defining therebetween a'fluidflow path.

5. A sound attenuating structure as defined in claim 3, wherein thelateral cross section of said mat comprises an annulus having an outsidesurface parallel to the inside surface of said shell and an insidesurface defining a hollow interior of said mat.

6. A sound attenuating structure as defined in claim 5, wherein saidinterior surface of said mat defines an axial passage for flow of fluidsthrough said conduit.

7. A sound attenuating structure as defined in claim 6, furthercomprising means restricting fluid flow in said conduit to said axialpassage.

where f is the highest acoustic frequency to be attenu ated, f is thelowest acoustic frequency to be attenuated, and c is the soundpropagation velocity in the ambient medium.

9. A sound attenuating structure as defined in claim 3, furthercomprising:

partition means extending transversely through said mat and connected tosaid shell to divide said mat into successive sound absorption sectionsand to hold said sections in fixed relationship to said shell. 10. Asound attenuating structure as defined in claim 5, further comprising:

partition means extending through said mat and the annular interspatialzone between said shell and said mat and having means defining acentrally disposed opening to leave said axial passage open for fluidflow.

1. A sound attenuating structure, comprising; a shell; a mat of sound-absorbing material disposed adjacent and parallel to said shell and spaced therefrom a distance substantially equal to one-fourth of the wave length of the highest frequency to be attenuated; and means for holding said mat parallel to said shell.
 2. A sound attenuating structure, comprising: a shell; a mat of sound absorbing material disposed adjacent to said shell and spaced therefrom a distance substantially equal to one-fourth of the wave length of the highest frequency to be attenuated, said mat having a thickness d having a value approximated by the expression: d ( c(f2- f1))/4.f1.f2 where f2 is the highest acoustic frequency to be attenuated, f1 is the lowest acoustic frequency to be attenuated, and c is the sound propagation velocity in the ambient medium.
 3. A sound attenuating structure, comprising: a shell forming the exterior wall of a conduit; a mat disposed interiorly of and adjacent to said shell and spaced from the interior surface thereof a distance substantially equal to one-fourth of the wave length of the highest frequency to be attenuated; and means for holding said mat parallel to said shell.
 4. A sound attenuating structure as defined in claim 3, wherein said mat forms a solid core filling the axial portion of said conduit, the mat and the interior surface of said shell defining therebetween a fluid flow path.
 5. A sound attenuating structure as defined in claim 3, wherein the lateral cross section of said mat comprises an annulus having an outside surface parallel to the inside surface of said shell and an inside surface defining a hollow interior of said mat.
 6. A sound attenuating structure as defined in claim 5, wherein said interior surface of said mat definEs an axial passage for flow of fluids through said conduit.
 7. A sound attenuating structure as defined in claim 6, further comprising means restricting fluid flow in said conduit to said axial passage.
 8. A sound attenuating structure as defined in claim 6, wherein: the thickness of said mat between said inside mat surface and said outside mat surface has a value approximated by the expression: d ( c(f2- f1))/4.f1.f2 where f2 is the highest acoustic frequency to be attenuated, f1 is the lowest acoustic frequency to be attenuated, and c is the sound propagation velocity in the ambient medium.
 9. A sound attenuating structure as defined in claim 3, further comprising: partition means extending transversely through said mat and connected to said shell to divide said mat into successive sound absorption sections and to hold said sections in fixed relationship to said shell.
 10. A sound attenuating structure as defined in claim 5, further comprising: partition means extending through said mat and the annular interspatial zone between said shell and said mat and having means defining a centrally disposed opening to leave said axial passage open for fluid flow. 